OK, so this is a seasonal flavor from Jeni's Splendid ice creams that isn't currently available. BUT, before you scream at me, maybe if we all write to them and ask them to bring it back this fall, they will. Because it is one of the most glorious things I've ever tasted. Also you could use David Lebovitz's sweet potato ice cream recipe from his awesome cookbook The Perfect Scoop, currently available on Scribd — but you're on your own with the marshmallows.